This invention relates to an exchange and, more particularly, to an exchange having a function for detecting a fault in an intra-office unit and a function for executing prescribed control to deal with the fault.
There is increasing demand not only for audio communication and data communication but also for multimedia communication in which moving pictures are transmitted as well as audio and data. B-ISDN (broadband-ISDN) switching technology, which is based upon an asynchronous transfer mode (ATM), is being put to practical use as a means for realizing broadband communication for multimedia communication. In an ATM transmission system, all information is transferred at high speed upon being converted to fixed information referred to as cells. More specifically, in an ATM transmission system, logical links are multiplexed on a physical line to thereby allocate the line to a plurality of calls. Dynamic image data or audio data from a terminal corresponding to each call are broken down into fixed-length information units (referred to as "cells"), and the cells are transmitted over a line sequentially to realize multiplexing.
ATM network technology has been developed for the purpose of constructing the next generation of B-ISDNs (Broadband Integrated Services Digital Networks), and this technology is now being put into use. FIG. 15 is a block diagram illustrating the configuration of an ATM switching system. Shown in FIG. 15 are subscriber interfaces (or line IFs) 11.sub.11 .about.11.sub.1n, 11.sub.21 .about.11.sub.2n, 11.sub.31 .about.11.sub.3n, 11.sub.41 .about.11.sub.4n connected to corresponding lines (transmission lines), multiplexer/demultiplexers 12.sub.1 .about.12.sub.4, an ATM switch unit 13, a system controller (processing unit) 14 and a maintenance terminal 15. The ATM switch unit 13 is connected to the plurality of multiplexer/demultiplexers 12.sub.1 .about.12.sub.4, switches input cells from certain multiplexer/demultiplexers and outputs the cells to prescribed multiplexer/demultiplexers. The multiplexer/demultiplexers 12.sub.1 .about.12.sub.4, which are connected to the pluralities of line interfaces 11.sub.11 .about.11.sub.1n, 11.sub.21 .about.11.sub.2n, 11.sub.31 .about.11.sub.3n, 11.sub.41 .about.11.sub.4n, respectively, multiplex uplink cells from a plurality of line interfaces IF and output the cells to the ATM switch unit 13. Furthermore, the multiplexer/demultiplexers 12.sub.1 .about.12.sub.4 demultiplex and output downlink cells, which arrive from the ATM switch unit 13, to the pertinent line interfaces.
The line interfaces 11.sub.11 .about.11.sub.4n, which are connected to the corresponding multiplexer/demultiplexers 12.sub.1 .about.12.sub.4, each extract an ATM cell (see FIG. 16A) mapped to the payload of a frame signal (e.g. a SONET frame) that has entered from the line and then subject the cell to predetermined processing (billing processing, UPC processing, OAM processing etc.) before outputting the cell to the multiplexer/demultiplexer. The multiplexer/demultiplexers 12.sub.1 .about.12.sub.4 multiplex the cells that enter from line interfaces, add routing information (tag information) TAG (FIG. 16B) onto the cells and then enter the cells into the ATM switch 13. The ATM switch unit switches a cell to a prescribed path by referring to this tag information TAG. The multiplexer/demultiplexers 12.sub.1 .about.12.sub.4 demultiplex the cells based upon the tag information and enter the cells into the prescribed line interfaces IF. The line interfaces 11.sub.11 .about.11.sub.4n map the ATM cells, which enter from the multiplexer/demultiplexers 12.sub.1 .about.12.sub.4, to the payload of a SONET frame and send the ATM cells to the line side. The system controller 14 controls the line interfaces 11.sub.11 .about.11.sub.4n, multiplexer/demultiplexers 12.sub.1 .about.12.sub.4 and ATM switch unit 13.
FIG. 16A is a diagram showing the structure of an ATM cell, and FIG. 16B is a diagram illustrating the structure of an intra-office cell onto which tag information has been added. A cell is composed of a cell header HD and a 48-byte payload PLD. The header HD includes (1) a generic flow control (GFC) used in flow control between links, (2) a virtual channel identifier (VCI) for call identifying purposes, (3) a virtual path identifier (VPI) for specifying a path, (4) payload type (PT), (5) cell loss priority CLP and (6) header error control (HEC). In the case of the intra-office cell, the header HD further includes (7) tag information TAG. RES represents reserve.
FIG. 17 is a block diagram illustrating the construction of a line interface for a SONET. One line interface (an OC3C interface) 11 is provided in correspondence with one set of uplink/downlink optical lines. A multiplexer/demultiplexer (MUX/DMUX) and ATM switch are shown at 12 and 13, respectively, a physical layer interface at 21 and an ATM layer interface at 22.
The physical layer interface 21 includes an optoelectric converter 21a, an electro-optic converter 21b and a physical layer termination (SONET termination) 21c. The physical layer interface 21 (1) separates and outputs cells that have been mapped to the payload of a SONET frame signal and, in addition, maps a cell that has entered from the side of the ATM switch to the payload of a SONET frame signal and sends the cell to the corresponding line, (2) executes overhead processing and (3) executes SONET/SDH end-point processing.
The line interface 11 further includes an ATM layer termination 22a which executes (1) STM-ATM conversion, (2) cell length conversion, (3) cell synchronization processing and (4) cell loop-back processing, and a UPC/NPC processor 22b which performs monitoring to determine whether the reported value of transmission capacity and the actual cell inflow quantity conform. When cells in excess of the reported value flow in, processing for discarding cells in contravention of the stipulation is executed. The line interface 11 further includes a billing processor 22c, which counts the number of passing ATM cells and creates billing data. An OAM processor 22d implements a fault management function for identifying and notifying of faults, and a performance management function for managing error rate and cell loss rate of user information cells.
FIG. 18 is a diagram showing the construction of the multiplexer/demultiplexer 12. The multiplexer/demultiplexer includes VCC setting units 31.sub.1 .about.31.sub.4 provided for corresponding ones of the line interfaces 1b.sub.1 .about.1b.sub.4. Each VCC setting unit has a VC converter 31a for adding a routing tag onto the header of an input cell and replacing the input VCI contained in the header with an output VCI, and a VC conversion table 31b for storing the routing tags and the output VCIs. The multiplexer/demultiplexer further includes a multiplexer 33 for multiplexing cells, which enter from the plurality of line interfaces 11.sub.1 .about.11.sub.n, and entering the cells into the ATM switch SW, and a demultiplexer 34 for demultiplexing cells, which have been switched by the ATM switch 13, and entering the cells into prescribed line interfaces 11.sub.1 .about.11.sub.4 upon referring to the tag information TAG.
Thus, in ATM transmission, all information is sent and received in the form of cells of fixed length. This makes possible high-speed data communication and high-quality image communication by hardware. However, if cell loss or cell error (bit error) occur owing to hardware failure in the ATM exchange, the high-speed communication of data and high-quality communication of images becomes impossible. Consequently, in a case where cell loss or cell error has taken place in an ATM exchange, it is necessary to detect the same, specify the location at which the cell loss or cell error occurred and effect recovery promptly.
With a large-capacity ATM exchange, however, the apparatus is highly complex owing to ATM switches and multiplexer/demultiplexers arranged in multiple stages. Accordingly, a problem which arises is that it is difficult to determine in which intra-office unit cell loss and cell error (bit error) have occurred.
Further, in order to conclude that a unit is a faulty unit, the conventional practice is for a serviceman to run a test on the equipment. However, specifying a suspect piece of equipment while verifying the number of cells that pass through each unit is time consuming. Furthermore, a detailed inspection for cell error cannot be performed unless special measuring equipment is used.
Further, equipment is replaced or a changeover is made to standby equipment depending upon the circumstances under which a fault occurs. However, since specifying the fault takes time, as mentioned above, the replacement or changeover cannot be performed promptly. This has a major effect upon the ATM subscribers.